Prediction of the ultimate behaviour of tubular joints in offshore jacket structures using nonlinear finite element methods

Abstract

Tubular joints are of great importance in offshore jacket structures. This thesis examines the ultimate state behaviour of tubular joints in offshore structures. In particular, the validity of a nonlinear finite element method was investigated and it was subsequently used to determine the ultimate load behaviour of a range of tubular joints. A geometrically nonlinear, eight node isoparan-letric shell finite element program is developed which allows six degrees of freedom per node. The material laws in the model include elastic and elastoplastic multilaver solution with integration across the thickness. Strain hardening elfects can be included. The nonlinear solution strategies are based on the Newton-Raphson Method. The load is applied in increments where for each step, equilibrium iterations are carried out to establish equilibrium, subject to a given error criterion. To cross the limit point and to select load increments, iterative solution strategies such as the arc length and autoniatic load increments method are adopted. To analyse tubular joints, a simple inesh generator has been developed. Struc- Cural symmetry is exploited to reduce the number of elements. The hibular joijil. is divided into a few regions and by means of a blending function. each region is discret, ised into a joints have been analysed using this finite element method. The numerical results have been compared with experimental tests undertak- en by the Wimpey Offshore Laboratory using large scale specimens.

Finally, the applicabiliy of the nonlinear finite element developed here is briefly discussed all potential areas of research in the ultimate behaviour of tubular joints are proposed.